Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Applications for fuel cells include battery replacement, mini- and microelectronics such as portable electronic devices, sensors such as gas detectors, seismic sensors, and infrared sensors, electromechanical devices, automotive engines and other transportation power generators, power plants, and many others. One advantage of fuel cells is that they are substantially pollution-free.
Electrochemical fuel cells convert fuel and oxidant fluid streams to electricity and reaction product. Solid polymer electrolyte fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. An electrocatalyst is typically disposed at each membrane/electrode layer interface to induce the desired electrochemical reaction.
The electrode substrate typically comprises a sheet of porous, electrically conductive material, such as carbon fiber paper or carbon cloth. The layer of electrocatalyst is typically in the form of finely committed metal, such as platinum, palladium, or ruthenium, and is disposed on the surface of the electrode substrate at the interface with the membrane electrolyte in order to induce the desired electrochemical reaction. In a single cell, the electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
The fuel stream directed to the anode by a fuel flow field migrates through the porous anode and is oxidized at the anode electrocatalyst layer. The oxidant stream directed to the cathode by an oxidant flow field migrates through the porous cathode and is reduced at the cathode electrocatalyst layer.
Electrochemical fuel cells can employ gaseous fuels and oxidants, for example, those operating on molecular hydrogen as the fuel and oxygen in air or a carrier gas (or substantially pure oxygen) as the oxidant. In hydrogen fuel cells, hydrogen gas is oxidized to form water, with a useful electrical current produced as a byproduct of the oxidation reaction. A solid polymer membrane electrolyte layer can be employed to separate the hydrogen fuel from the oxygen. The anode and cathode are arranged on opposite faces of the membrane. Electron flow along the electrical connection between the anode and the cathode provides electrical power to load(s) interposed in circuit with the electrical connection between the anode and the cathode. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:Anode reaction: H2 →2H++2e−Cathode reaction: ½O2+2H++2e−→H2O
The catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion-exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing gaseous fuel stream from the oxygen-containing gaseous oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. Hydrogen fuel cells are impractical for many applications, however, because of difficulties related to storing and handling hydrogen gas.
Organic fuel cells can prove useful in many applications as an alternative to hydrogen fuel cells. In an organic fuel cell, an organic fuel such as methanol or formic acid is oxidized to carbon dioxide at an anode, while air or oxygen is simultaneously reduced to water at a cathode. One advantage over hydrogen fuel cells is that organic/air fuel cells can be operated with a liquid organic fuel. This diminishes or eliminates problems associated with hydrogen gas handling and storage. Some organic fuel cells require initial conversion of the organic fuel to hydrogen gas by a reformer. These are referred to as “indirect” fuel cells. The presence of a reformer increases cell size, cost, complexity, and start up time. Other types of organic fuel cells, called “direct,” eliminate these disadvantages by directly oxidizing the organic fuel without conversion to hydrogen gas. To date, fuels employed in direct organic fuel cell development methanol and other alcohols, as well as formic acid and other simple acids.
In fuel cells of this type the reaction at the anode produces protons, as in the hydrogen/oxygen fuel cell described above, however the protons (along with carbon dioxide) arise from the oxidation of the organic fuel, such as formic acid. An electrocatalyst promotes the organic fuel oxidation at the anode. The organic fuel can alternatively be supplied to the anode as vapor, but it is generally advantageous to supply the organic fuel to the anode as a liquid, preferably as an aqueous solution. The anode and cathode reactions in a direct formic acid fuel cell are shown in the following equations:Anode reaction: HCOOH+→2H++CO2+2e−Cathode reaction: ½O2+2H++2e−→H2OOverall reaction: HCOOH+½O2→CO2+H2O
The protons formed at the anode electrocatalyst migrate through the ion-exchange membrane from the anode to the cathode, and at the cathode electrocatalyst layer, the oxidant reacts with the protons to form water.
One obstacle to the widespread commercialization of direct fuel cell technology is fuel crossover from the anode to the cathode through the typical proton exchange membranes (such as, for example, perfluorosulfonic acid membranes, of which Nafion® is a commercial brand). Fuel crossover lowers fuel utilization efficiency and also adversely affects the cathode (oxygen-reduction electrode), resulting in poor fuel cell performance. Fuel crossover also reduces the run time for a given amount of fuel and creates excess heat and water on the cathode side of the cells.
Fuel that crosses over avoids reaction at the anode, and thus cannot be exploited for electrical energy. This limits cell efficiency. An additional problem related to crossover is poisoning of the cathode. As fuel crosses over the polymer membrane to the cathode side, it adsorbs onto the cathode catalyst and thereby blocks reaction sites. Efficiency of the cell is thereby reduced. A proposed solution to this problem has been to provide additional catalyst. This adds expense; however, particularly when considering that costly precious and semi-precious metal catalysts such as platinum are often employed.
Because of this high crossover, methanol and other alcohol fuel cells typically operate with a fuel concentration of no more than about 3-8% by weight. The use of those dilute solutions creates additional problems. This low fuel concentration requires relatively large amounts of ultra-pure water, typically provided through recycling systems including pumps and filters. Also, the concentration of the fuel should be closely monitored and controlled, with the result that sensors and controllers may be required. This peripheral equipment adds cost, complexity, weight, and size to direct organic fuel cells. This required peripheral water management equipment substantially limits the usefulness of direct methanol fuel cells for applications where size and weight become important. For portable, miniature, and microelectronics applications, for example, the size, weight, and complexity of the required peripheral equipment make use of direct fuel cells impractical.
Further, the dilute solutions freeze and expand at temperatures potentially encountered in many fuel cell applications, with portable devices for use outside as an example. The expansion can lead to device failure. Conduit et al. U.S. Pat. No. 6,528,194 teaches that the freezing can be avoided by circulating heated fluid through the fuel tank when the fuel cell is not operating. However, that wastes power and adds complexity.
Proton exchange membranes with low liquid fuel permeation (or low crossover) can allow the use of liquid fuels with high concentration. The ability to use a fuel in a higher concentration increases the energy density available from a direct fuel cell, which is particularly attractive for portable electronic applications such as cellular telephones, personal digital assistants, laptop computers, and handheld gaming platforms. The crossover of formic acid has generally been demonstrated to be lower than the crossover of methanol. Nonetheless, the use of formic acid concentrations higher than 10M could also lead to an unacceptable level of fuel loss with typical Nafion® proton-conducting membranes.